1. Field of the Invention
The present invention generally relates to footwear, more particularly, to footwear structures and manufacturing methods for forming footwear structures by thermoforming one or more layers of material.
2. Discussion of the Background
A typical athletic shoe includes an upper, a midsole, and an outsole. The upper covers and protects the instep, heel, and side portions of the foot and is commonly constructed of leather or other natural or synthetic materials (e.g., nylon). The particular sport for which the athletic shoe is to be utilized often dictates the material or materials used to construct the upper. For example, for the upper of a basketball shoe, it is often desirable to utilize a heavy material such as leather because leather provides more support to the wearer's foot and ankle than canvas or nylon. A running shoe upper, however, might be formed almost entirely of a synthetic material because such a material is relatively lightweight, breathable, and easy to clean. However, a wide variety of materials or combinations of materials might be selected for a particular shoe design based upon factors such as cost, current styles and trends, and the ability to obtain the desired performance requirements with certain materials.
The midsole lies between the upper and the outsole and is provided mainly to cushion the heel and forefoot of the wearer. Synthetic materials such as polyurethane (PU), ethyl vinyl acetate (EVA), and polyester ethyl vinyl acetate (PEEVA) are commonly used to form the midsole. The midsole can be formed in one or more pieces and can also include a wedge or cushioning insert disposed beneath the heel of the wearer to effectively increase the amount of cushioning. During assembly, the midsole is typically bonded, either by cement or by fusion, to an inner sole assembly (or “sockliner”) of the shoe.
The outsole comes into direct contact with the ground and is commonly molded from an abrasive-resistant material, such as rubber. To provide traction to the wearer, the outsole includes geometries of protrusions and recessions designed to increase friction between the outsole and the contacting ground. Such geometries are chosen based on the particular activities that the shoe is intended to be used for. The outsole is bonded or adhered to the bottom surface of the midsole to complete the shoe unit. It can be difficult to precisely define the terms sole and midsole, because the terms are not always used uniformly, and the same or similar components could be considered as part of a sole or as part of a midsole. For example, sometimes the term “sole” is used to encompass both sole and midsole components. As used herein, the term “sole” refers to the outermost portion or layer of the shoe which contacts the ground in use, while “midsole” refers to a layer or layers above the sole. The term “sole assembly” is used generically to refer to one or more sole and/or midsole components, and thus, a sole assembly might or might not include a sole or a midsole. Thus a “sole assembly” could include only midsole components, only sole components, or both sole and midsole components. Where the “sole assembly” as used in the specification and appended claims is intended to specifically include certain components, specific reference is made thereto.
A current process for molding an upper involves backing a composite plastic component with a foam material and then pressure-forming the plastic and the foam into a desired shape using heat and high pressure. A variation of this foam-backing process involves pre-assembling layers of materials and then forming the assembled layer into a finished component.
A known process for manufacturing sole assemblies involves a twin-sheet thermoforming process, as described in U.S. Pat. No. 5,976,451 to Skaja et al., the disclosure of which is hereby incorporated by reference in its entirety. In this process, a single footwear structure is formed by the combination of two separate material layers, which are separately thermoformed on respective non-mating molds and subsequently attached.
The process of twin-sheet thermoforming includes a step of first heating each material layer to a forming temperature, which is a temperature at which a material is pliable enough to be shaped into a desired form. Then, the softened material layer is positioned on a mold having a desired shape. The two molds used in this process are not mating male and female molds, but are shaped to separately create different portions of a final component which are combined after the portions are formed. The positioning step includes securing the edges of each material layer to its respective mold, e.g., by clamps.
Each material layer then undergoes a drawing step, in which the material layer is vacuum-molded against the mold. The mold is apertured such that a negative or vacuum pressure is created through the forming surface when a drawing device is activated to create a vacuum in the mold. Preferably, each material layer has a hot tensile strength adequate to allow the material layer to stretch uniformly onto and around the mold. External or positive air pressure can be applied to the material layer opposite to the forming surface to assist in forcing the material layer firmly into the forming surface.
As a result of the drawing step, each material layer assumes the shape of the mold that it is positioned over. Each material layer is then allowed to cool on its mold to a set temperature, at which the material layer hardens sufficiently to permit removal of the material layer from the mold without a resulting deformation. When the material layer belongs to a particular class of materials, as discussed below, the assumed shape is permanent under normal usage. The two formed layers are then attached to one another (e.g., by gluing or welding) and trimmed to a desired component shape. Alternatively, the two material layers can be combined while they are still positioned on their molds. This is accomplished by bringing the molds toward each other until the material layers contact one another, while both material layers are at a temperature allowing adhesion by such resulting pressure.
Both of the above-described processes are constrained in several ways. For example, each process requires two molds, which must be separately heated and cooled. In addition to operating costs, mold costs are very high due to dual-mold requirements, heat dissipation, and high-cost mold materials. For durability reasons, molds used in twin-sheet processing must be made of steel, which is relatively expensive to cut and is difficult to handle. This cost is then multiplied by the number of sizes that will be manufactured for a given shoe design. Also, cycle times using a two-mold, twin-sheet process are typically between 60 and 90 seconds, which is not conducive to mass production. Further, the cost-per-unit is often prohibitively high due to the significant costs of multiple materials, the combining/assembly process and associated labor, and the costs associated with defects.
Further, with respect to the foam-backing process, molded uppers made using this method are relatively heavy, due to the combination of materials used. In addition, the molded uppers cannot be made breathable by the above-described process itself; additional processes are required to create apertures in the molded uppers for this purpose. Such additional processes are expensive, time-consuming, and often ineffective or less effective than desired. Further, the described process does not allow for the creation of surface textures on the molded uppers, as the molding surfaces can not be realistically processed to include such texturing, due to the hardness of the mold materials. For the same reason, graphics desired on the uppers are limited to the outer-most surface and have no three-dimensional quality. Typically, any desired aesthetic or structural attachments are limited to those of the stitch-on or adhesive variety. Moreover, undercuts in an upper are not possible due to the dual-mold requirement.
Twin-sheet manufacturing processes can also be disadvantageous in not providing optimal combinations of structural support, cushioning, and flexibility, while allowing for efficient manufacturing, factoring into consideration costs associated with materials, tooling, and labor. Also, this process does not provide for the integration of additional insert material layers during the thermoforming process such that the insert material layers are heat-bonded and vacuum-bonded to the formed component. Further, twin-sheet processing does not allow for the use of male-shaped molds in an efficient way.